“It was a terrible accident,” Puebla Archbishop Victor Sanchez said in his homily about the accident that injured another two nuns.
“I ask everyone to pray for them so that God takes them in his glory and help the families cope.”
The nuns, aged between 20 and 46, belonged to the Order of the Most Holy Savior and Saint Bridget.
They were returning to their convent late Saturday in Puebla after spending a few days in a retirement home in Ciudad Serdan.
Their bus fell into an 80-meter (260-foot) ravine).
The two surviving nuns were recovering in hospital from serious injuries.
The accident is under investigation by the state attorney and the cause remains unknown.